Radioactivity (AQA A-Level Physics): Revision Notes
Applications of radioisotopes
Although radioactive emissions pose risks, certain radioactive isotopes (radioisotopes) have valuable applications in medicine and industry. The radioisotopes selected for practical use are chosen through careful risk-benefit analysis to ensure their advantages outweigh the potential hazards.
The selection of radioisotopes for practical applications involves evaluating both the benefits they provide and the potential risks they pose. This ensures that only isotopes with significant advantages and manageable safety protocols are used in medical and industrial settings.
Medical applications
Cancer treatment with alpha emitters
Radium-233 is an alpha-emitting isotope used in targeted cancer therapy. Small quantities of radium-233 can be injected directly into tumorous tissue where the alpha particles destroy cancer cells. Alpha radiation is particularly effective for this purpose because:
- Alpha particles have high ionising power, causing maximum damage to cellular DNA
- Alpha particles have very short range in tissue, limiting damage to surrounding healthy cells
- The radiation is delivered directly to the site requiring treatment
The combination of high ionising power and short range makes alpha emitters ideal for targeted cancer therapy. The alpha particles can effectively destroy cancer cells while minimising damage to nearby healthy tissue, making treatment more precise and safer for patients.
Treatment of thyroid cancer
Iodine-131, a beta-emitting isotope, is used specifically for treating thyroid cancer. The thyroid gland naturally concentrates iodine from the bloodstream, so radioactive iodine-131 accumulates preferentially in thyroid tissue. The beta radiation then damages the cancerous cells while having minimal effect on other body tissues.
This selective uptake mechanism makes iodine-131 particularly effective for thyroid treatments. The body's natural iodine concentration process in the thyroid ensures the radioactive isotope reaches exactly where it's needed, reducing exposure to other organs.
Diagnostic tracers with beta-plus emitters
Beta-plus emitters such as carbon-11, nitrogen-13, and fluorine-18 serve as diagnostic tracers. These isotopes are chemically incorporated into compounds that the body naturally uses in metabolic processes. The procedure works as follows:
- The radioactive compound is administered to the patient
- The compound is absorbed and concentrated in specific organs or tissues
- When the radioactive nuclei decay, they emit positrons (beta-plus particles)
- The positrons annihilate with electrons, producing pairs of gamma ray photons
- A PET (Positron Emission Tomography) scanner detects these gamma rays
- Computer analysis creates detailed images showing the distribution and concentration of the tracer
This technique allows doctors to visualise organ function and detect abnormalities such as tumors, which often show different uptake patterns compared to healthy tissue.
PET Scanning Advantage
The beauty of PET scanning lies in its ability to reveal not just structure, but function. By tracking how the body uses the radioactive tracer, doctors can identify metabolic abnormalities that might not be visible on traditional imaging, often detecting diseases at earlier stages.
Imaging with low-energy gamma sources
Low-energy gamma-emitting isotopes are also used as tracers for internal imaging. These work similarly to beta-plus emitters:
- The radioactive isotope is combined with a compound taken up by specific body tissues
- The patient receives the compound, which distributes according to normal body processes
- The gamma radiation emitted can penetrate body tissues to reach external detectors
- A gamma camera detects the radiation and tracks the movement and distribution of the compound
- This produces images revealing both the structure and function of internal organs
The low energy of these gamma sources ensures the radiation dose to the patient is minimised while still providing sufficient signal for imaging.
Sterilisation of medical equipment
Gamma radiation is highly effective for sterilising medical equipment. The high-energy photons can:
- Penetrate through packaging and equipment materials
- Damage the DNA of bacteria and viruses, killing microorganisms
- Sterilise items without exposing them to heat or chemicals that might cause degradation
Heat-Sensitive Equipment
Gamma sterilisation is particularly crucial for medical items that cannot withstand traditional heat sterilisation methods. This includes many modern plastics, electronics, and delicate surgical instruments that would be damaged by high temperatures.
Industrial applications
Static electricity elimination
Polonium-210, an alpha emitter, functions as a static eliminator in industrial settings. During manufacturing processes such as paper, plastics, and textiles production, static electricity can build up on materials. The alpha particles from polonium-210 ionise the surrounding air, creating ions that neutralise the static charges on the materials. This prevents:
- Materials sticking together due to static attraction
- Dust accumulation on products
- Potential sparks that could ignite flammable materials
The ionisation of air creates a conducting pathway that allows static charges to dissipate harmlessly. This is essential in industries where static buildup could ruin products, contaminate surfaces, or even create dangerous sparking conditions in environments with flammable materials.
Thickness monitoring and control
Strontium-90, a beta source, is used to monitor and control the thickness of manufactured sheet materials including paper, plastic films, aluminium foil, and thin steel sheets. The system operates through the following mechanism:
- The beta source is positioned on one side of the moving sheet material
- A detector is placed on the opposite side to measure transmitted radiation
- Thicker material absorbs more beta particles, reducing the detected count rate
- Thinner material absorbs less radiation, increasing the detected count rate
- The detector connects to a computer that continuously monitors the radiation level
- The computer provides feedback to control the pressure applied by rollers
- This automated system maintains correct thickness throughout production
Worked Example: Thickness Control in Action
Consider a paper mill producing paper with a target thickness of mm:
Initial reading: Detector measures counts per second with correct thickness
Scenario 1 - Paper too thick:
- Detector measures counts per second (fewer particles transmitted)
- Computer signals rollers to increase pressure
- Paper thickness reduced back to target
Scenario 2 - Paper too thin:
- Detector measures counts per second (more particles transmitted)
- Computer signals rollers to decrease pressure
- Paper thickness increased back to target
This continuous feedback maintains consistent product quality automatically.
The beta particles from strontium-90 are ideal for this application because they have sufficient penetration to pass through thin materials but are significantly absorbed by small thickness variations.
Industrial tracers for leak detection
Gamma-emitting isotopes serve as tracers in industrial applications, particularly for detecting leaks in pipelines. The method involves:
- Introducing a small quantity of radioactive gas into the pipeline
- Using radiation detectors to measure gamma intensity at ground level above the pipeline
- Identifying leak locations by detecting increased gamma radiation at the surface
- The gamma radiation can penetrate through soil and pipe materials to reach the detector
Non-Invasive Detection Advantage
This technique allows engineers to locate leaks without the need for expensive and time-consuming excavation of entire pipeline lengths. The gamma radiation's penetrating ability means leaks can be detected from the surface, saving significant costs and reducing environmental disruption.
Radiation monitoring and safety
Dosimeter badges
People who work with ionising radiation must wear a radiation dosimeter badge to monitor their cumulative radiation exposure. The badge:
- Records the total amount of radiation absorbed by the wearer over time
- Provides a measurement of the radiation dose in sieverts (Sv)
- Takes account of the relative biological effects of different types of ionising radiation
- Ensures workers do not exceed safe exposure limits
- Allows employers to maintain safety records and identify potential overexposure
Worker Safety is Paramount
The sievert is the SI unit that quantifies the biological effect of absorbed radiation, accounting for both the amount of energy deposited and the type of radiation involved. Regular monitoring through dosimeter badges is a legal requirement in most countries and is essential for protecting workers' long-term health.
Key Points to Remember:
- Alpha emitters (radium-233, polonium-210) are used for direct cancer treatment and static elimination due to their high ionising power and short range
- Beta emitters (iodine-131, strontium-90) are suitable for thyroid treatment and thickness monitoring because of their moderate penetration
- Beta-plus emitters (carbon-11, nitrogen-13, fluorine-18) combined with PET scanning provide detailed functional imaging of organs
- Gamma sources are versatile, used for imaging tracers, sterilisation, and industrial leak detection due to their high penetration
- Dosimeter badges measure cumulative radiation exposure in sieverts, protecting workers from excessive doses
Remember: The choice of radioisotope depends on matching the radiation type's properties (ionising power, penetration, range) to the specific application's requirements.